My friend pulls a 6800 lb (dry) KZ fifth wheel with his EcoBEAST, but he hasn't pulled it in the mountains yet.
But he said when he went through one of the big river valleys here, it only downshifted to 5th, and never lost one mph going up the other side. The grades are not posted, but from my experience in the mountains, I would estimate it at 7% minimum.
And he has lots of crap loaded into his camper, so I would say he is well over 8000 lbs gross. And he is getting close to 30 mpg (Canadian) empty, as long as he sticks to the speed limit. It does seem to be getting better as he gets some miles on the truck.
But it does like the gas when pulling, and he is down in the 12 mpg range, which would be about 10 on the US gallon.
He simply loves his new EcoBeast.